i894.] THE BRITISH NATURALIST. 13 



fall within the five classes which are collectively known as Arthropods, 

 an Anglo-Greek word indicating the most striking characteristic of this 

 group, the segmentation not merely of the body but of the limbs and 

 appendages. It may be easily imagined that there are great differences 

 in the breathing apparatus of animals so diverse as the centipede and 

 the dragon-fly, the louse and the lobster, the beetle and the butterfly, the 

 spider and the scorpion, all of which, and many more, despite their 

 great differences, present such striking resemblances of structure as to 

 cause systematic naturalists to group them together as forming one of 

 the most important divisions of the animal kingdom. 



The most common method of respiration in the Arthropods is by tracheae : 

 canals which open externally, usually by one orifice on each side of each 

 segment of the body behind the head, and which ramify internally, carry- 

 ing air to the blood which bathes the walls of these delicate air vessels ; 

 there are commonly two longitudinal canals running on either side of the 

 body and connecting the larger tracheae ; the mouths of which are often 

 protected by valves ; this arrangement of the air vessels presents a 

 striking and scarcely accidental resemblance to the system ot water 

 vessels in many of the worms ; in fact the breathing apparatus of the 

 highest insect, consisting as it does of two main lateral air trunks with 

 innumerable ramifications from them, carrying oxygen into every part of 

 the body, may be regarded as the last and highest form of the series 

 which begins in the lowest worms. 



Though many insects live in water, yet none possess gills, either in the 

 larval or adult form. Water beetles, as probably all readers of this 

 magazine are aware, take air down under their elytra, and when that is 

 exhausted, come to the surface for a fresh supply ; the so-called water 

 scorpion (Nepa) possesses two long tubular processes which communicate 

 with its tracheal system. By this apparatus it is enabled to procure fresh 

 air without absolutely coming to the surface ; it only needs to come so near 

 to the top that the tips of its air processes project above the level of the 

 water and permit the air to pass down into the tracheae. This absence 

 of gills is characteristic even of the aquatic larvae of insects, which 

 possess a complete tracheal system, but the mouths of the tracheae (stig- 

 mata or spiracles, as they are variously called) are closed. Some of the 

 segments of the body develop delicate processes, into which the branches 

 3f the tracheae enter ; these processes are often called tracheo-branchiae, 

 . 3ut the name is liable to mislead since they are in no sense branchiae or 

 jills. Gills are organs in which the blood is enabled to take up from the 

 . vater some of the oxygen held in solution, whilst the delicate processes 

 )f the aquatic larvae referred to, allow the passage of the oxygen into 

 heir own central tube, whence it is conveyed to the tracheae, and so to 

 he various parts of the body. 



(To be continued?) 



